1. Field of the Invention
The present invention relates to an infectious bursal disease virus (IBDV) strain comprising a variant structural protein VP2 that can be used to provide protection against a virulent challenge by classic and variant strains of IBDV.
2. Description of the Related Art
Infectious bursal disease virus (IBDV), a member of the Birnaviridae family, is the causative agent of a highly immunosuppressive disease in young chickens (Kibenge et al., 1988). Infectious bursal disease (IBD) or Gumboro disease is characterized by the destruction of lymphoid follicles in the bursa of Fabricius. In a fully susceptible chicken flock of 3-6 weeks of age the clinical disease causes severe immunosuppression, and is responsible for losses due to impaired growth, decreased feed efficiency, and death. Susceptible chickens less than 3 weeks old do not exhibit outward clinical signs of the disease but have a marked infection characterized by gross lesions of the bursa.
IBDV is a pathogen of major economic importance to the nation and world's poultry industries. It causes severe immunodeficiency in young chickens by destruction of precursors of antibody-production B cells in the bursa of Fabricius. Immunosuppression causes increased susceptibility to other diseases, and interferes with effective vaccination against Newcastle disease, Marek's disease and infectious bronchitis disease viruses.
The capsid of the IBDV virion consists of several structural proteins. As many as nine structural proteins have been reported but there is evidence that some of these may have a precursor-product relationship. The designation and approximately molecular weights of the viral proteins (VP) are as shown below.
Viral ProteinMolecular WeightVP190 kDaVP241 kDaVP332 kDaVP428 kDaVP5 (NS)17 kDa
The IBDV genome consists of two segments of double-stranded (ds) RNA that vary between 2827 (segment B) to 3261 (segment A) nucleotide base pairs. The larger segment A encodes a 110-kDa precursor protein in a single large open reading frame (polyprotein ORF) which is cleaved by autoproteolysis to form the mature viral proteins VP2, VP3 and VP4 (Hudson et al., 1986). VP2 and VP3 are the major structural proteins of the virion. VP2 is the major host-protective immunogen of IBDV, and contains the antigenic regions responsible for the induction of neutralizing antibodies (Azad et al., 1987). A second open reading frame (ORF) encodes a protein (VP5), a 15-17 kDa arginine-rich nonstructural protein (NS), which precedes and partially overlaps the major polyprotein ORF. Although this protein is not present in the virion, it is detected in infected cells. The genomic segment B encodes VP1, a 90-kDa minor internal protein, which is the virion-associated RNA-dependent RNA polymerase
In U.S. Pat. No. 6,231,868, the present inventor showed that the NS protein of IBDV (VP5) plays a role in viral pathogenesis by constructing a cDNA clone of IBDV segment A, wherein the initiation codon of the NS gene was mutated to a stop codon. Using the reverse genetics system, a wild-type IBDV was generated, as well as a mutant IBDV that lacked the expression of the NS protein. The properties of the recovered wild-type IBDV and mutant IBDV in cell culture were compared and their pathological function in the natural host evaluated. It was shown that the mutated IBDV, that lacked the expression of the NS protein, was less pathogenic than the wild type and that there was significant reduction in damage to the bursa of Fabricius.
It has been demonstrated that the VP2 protein is the major host protective immunogen of IBDV, and that it contains the antigenic region responsible for the induction of neutralizing antibodies. This region containing the neutralization site has been shown to be highly conformation-dependent. The VP3 protein has been considered a group-specific antigen because it is recognized by monoclonal antibodies directed against it from strains of both serotype I and II viruses. The VP4 protein appears to be a virus-coded protease that is involved in the processing of a precursor polyprotein of the VP2, VP3 proteins.
In the past, control of IBDV infection in young chickens has been achieved by live vaccination with avirulent strains, or principally by the transfer of maternal antibody induced by the administration of live and killed IBDV vaccines to breeder hens. Unfortunately, in recent years, virulent variant strains of IBDV have been isolated from vaccinated flocks in the United States. The use of a select panel of monoclonal antibodies (Mabs), raised against various strains of IBDV, has led to the identification of naturally occurring GLS, DS326, RS593 and Delaware variant viruses in the United States. Substantial economic losses have been sustained due to the emergence of these antigenic variants. These variant strains are antigenically different from the classic strains of IBDV most typically isolated before 1985, and lack epitope(s) defined by neutralizing Mabs B69 and R63 (Snyder et al., 1988a; Snyder et al., 1988b; Snyder et al., 1992). Since the appearance of these variant strains in the field, many commercially available live and killed vaccines for IBDV have been reformulated in an attempt to better match the greater antigenic spectrum of viruses recognized to be circulating in the field. For example, some companies have developed individual vaccines against GLS and Delaware strains of IBDV that are characterized by their reactivity with neutralizing Mabs 57 and 67, respectively.
Thus, it would be advantageous to develop a vaccine comprising a structural antigenic viral protein, which provides wider protection for multiple strains of IBDV by inducing antibody production directed at different strains of IBDV.